Continuous casting mould

ABSTRACT

A continuous casting mould for casting thin slabs has, at its inlet end, a mould cavity which has generally rectangular side regions joined by a central pouring region. At least part of the walls of the pouring region are of arcuate form and over at least part of the depth of the mould the radii of curvature of the arcuate parts progressively increase.

This invention relates to continuous casting of metal and, inparticular, to a continuous casting mould for casting workpieces in theform of thin slabs.

EP-A-0149734 and U.S. Pat. No. 4,721,151 both disclose a continuouscasting mould of the type to which the present invention relates. Bothpublications disclose a continuous casting mould having a cavityextending from the inlet end to the outlet end of the mould and thecavity has a cross-section, normal to the lengthwise direction,comprising two side regions of generally rectangular form spaced apartby a central region. In the lengthwise direction of the cavity from theinlet end thereof, the central region is enlarged to form a pouringregion into which, in use, the feed tube extends.

In the European publication, the width of the central region between thetwo side regions is not constant and the pouring region reducesconsiderably in width along its length.

With such a mould, tensile strains are applied to the solid/liquidinterface of the inside of the metal shell as it passes down through themould. These tensile strains can be detrimental when their magnitude islarge enough to open the grain boundaries which allows interdendriticpenetration by the liquid solute which can cause detrimentalsegregation. These strains may also create internal cracks.

In the U.S. publication, the width of the central region between the twoside regions is generally constant and the pouring region is of the samewidth along its length. However, the opposite walls of the pouringregion which separate the side regions are constituted either entirelyor partially of straight flat walls. It has been found that the use offlat walls reduces the area over which strand deformation occurs andincreases the possibility of tensile stress occurring in the strand orincreases the degree of compression required to cancel the tensilestress.

According to the present invention there is provided a continuouscasting mould having a cavity extending in a lengthwise direction fromthe inlet end to the outlet end of the mould and which cavity has across-section, normal to the lengthwise direction, comprising two sideregions of generally rectangular form spaced apart by a central regionof generally constant width which does not reduce and in the lengthwisedirection of the cavity from the inlet end thereof the central region isenlarged to form a pouring region; characterised in that the oppositewalls of the pouring region which separate the side regions are entirelyof arcuate form and over at least part of the lengthwise direction ofthe pouring region the radii of curvature of the arcuate wallsprogressively increase.

A containment zone may be positioned beneath the casting mould and thepouring region may extend into the containment zone.

In order that the invention may be more readily understood, it will nowbe described, by way of example only, with reference to the accompanyingdrawings, in which:

FIGS. 1 and 2 are diagrammatic front and sectional side elevations,respectively, of a continuous casting mould in accordance with oneembodiment of the invention;

FIGS. 1A through 1D comprise, respectively, cross sectionalillustrations taken along lines 1A--1A through 1D--1D, respectively, ofFIG. 1;

FIGS. 3 and 4 are diagrammatic front and sectional side elevations,respectively, of a continuous casting mould in accordance with a secondembodiment of the invention;

FIGS. 3A through 3D comprise, respectively, cross sectionalillustrations taken along lines 3A--3A through 3D--3D, respectively ofFIG. 3; and

FIG. 5 is a diagrammatic sectional side elevation of a continuouscasting mould in accordance with a third embodiment of the invention.

Referring now to the figures, a continuous casting mould for castingthin metal slabs, or thick strip, is indicated by reference numeral 1.The mould is conveniently of copper alloy and it is cooled in aconventional manner by means (not shown).

In each embodiment, a mould cavity 2 extends from the inlet to theoutlet end of the mould. There may be additional cavities (not shown) inthe mould. The mould is oscillatable in the direction of the length ofthe mould cavity by means (not shown).

In use, the mould is positioned above a containment zone 3, which mayconsist of grids and rollers 4 and which supports the cast slab whilstit is cooled and the shell of the slab thickens. A refractory feed tube5 extends into the inlet end of the mould cavity and molten metal passesthrough the tube into the mould.

In each embodiment, the cavity 2 in its widthwise direction comprisestwo side regions 7 which are rectangular and a central portion 8 whichis of generally constant width. At the inlet end of the mould, thecentral region is enlarged to form a pouring region which accommodatesthe ceramic feed tube 5 so that adequate clearance exists between themould walls and the outside of the tube 5. At least part of the enlargedcentral pouring region of the mould cavity constitutes a deformationarea 9 to deform that part of the shell formed in it as the shell ismoved in the direction of discharge. The deformation area is shaded inFIGS. 1 to 4. In the FIGS. 1 and 2 and the FIG. 5 embodiments it extendsto a position above the bottom outlet end of the mould. In the FIGS. 3and 4 embodiment it extends to the outlet end of the mould. Thedeformation area may commence at the inlet end or at a position inwardlyof the inlet end.

Referring now to FIGS. 1 and 2, the centre portion 8 has walls which areof arcuate form. The arcs are of ever increasing radius R which increaseto infinity at depth L. Over the distance l₁, from the top of the mould,the radius remains as R₁ but, at l₂, the radius R₂ is such that R₂ >R₁whilst the length of the chord C remains constant. In practice, arclength will reduce slightly but chord length will be generally constantor with a slight increase. In this arrangement the mould discharge shapeis a rectangle and the strand issuing from the mould is a rectangularsolidifed shell with a liquid core, as shown by reference numeral 10. Insuch a system the solid/liquid interface of the strand shell isbasically subjected to continuous compressive stresses at the verticaledges of the shell deformation area 9. As the shell is cooled by themould wall it contracts setting up a stress in the outer layer. Alongthe bottom horizontal edge of area 9, where the radius R becomesinfinitive, the stresses remain compressive as the transition fromcurved to flat is a gradual continuous change.

The arrangement shown in FIGS. 3 and 4 has a shell deformation area 9which extends into the containment zone 3. The containment zone maycomprise a series of rollers 4, as shown, or support grids and rollers,any of which are profiled to the radius required, i.e., R₁, R₂, R₃,etc., where R₃ >R₂ >R₁.

The metal flowing into the mould may be in liquid or solid plus liquidstate. When the metal is in the solid plus liquid state, which is knownas slurry, the structure will be finer with smaller grains than castingwith temperature above the liquidus. A structure with such small grainsize will require less mechanical working to give a suitable structurein the final product as well as eliminating the possibility ofinterdendritic solutes causing detrimental segregation.

The forces applied to the shell are basically compressive resulting froma uniformly distributed load on an arc or arch whose ends areconstrained at the points where the ever increasing radius meets theplanar narrow parallel ends. The compressive stresses are lowered as theradii of curvature are increased and the rate of change is lowered.Thus, from the meniscus, the shell in the centre portion 8 of the cavitywill have two opposite vertical arcs which are continuously increasingin radius over a width which does not reduce as the strand passesthrough the caster. The narrow face support adjacent to the deformationzone will contain the forces resulting from the shell deformation.

In the arrangement shown in FIG. 5, the deformation zone commences atthe inlet end of the mould and finishes at a position above the outletend of the mould. The cross-section of the casting as it leaves theoutlet end of the mould is rectangular with a pasty core.

Instead of totally deforming the shell to a rectangular flat shape, thestrand may be allowed to totally solidify with a thicker centre sectionprior to either full or final deformation into a rectangular flatsection equal to or less than the thickness of the narrow mould ends byrolling or forging prior to being cut into usable lengths.

The strand support below the mould may initially be configured to followthe orientation of the mould or it may be configured so that thedirection of discharge is changed, e.g., the mould may be vertical andthe strand support may also be vertical or it may be curved immediatelybelow or at a discrete distance below the meniscus either inside oroutside the mould.

The feed tube can be manufactured from one of a number of refractorymaterials. These materials have different thermal characteristics whichwill affect the temperature of the liquid metal adjacent to the tube andthe melting of the mould lubricant, hence the surface quality of thecast product. The feed tube can also be of composite construction withdifferent materials along its length or through its thickness. In someinstances this can be a detrimental effect, especially where melting ofthe lubricant is inhibited or where the bridging between shell and tubemay occur. In order to minimise/overcome this, the mould material ischosen such that the thermal conductivity at the area adjacent to theceramic feed tube is different to give a different heat removal rate toother parts of the mould to enable the performance of the mould powderto remain relatively constant all around and down the mould. i

For example, an insert 16 (FIG. 1) may be placed in the centre of eachof the longer walls of the mould cavity, the length of the inserts beingapproximately equal to the length of the feed tube. The insert has alower thermal conductivity than the remaining parts of the mould andthis ensures that the metal is hotter and the shell remains thinner inthe vicinity of the feed tube. The insert may be a matrix of metal suchas copper with a ceramic such as silicon nitride or boron nitrideimpregnated in it.

The widthwise dimension of the mould cavity may be adjustable. Forexample, the walls of the mould which define the ends of the sideregions 7 of the cavity may be movable towards and away from each otherto adjust the length of the side regions and hence the width of thestrand cast in the mould.

We claim:
 1. A continuous casting mould having a cavity of substantiallyuniform width extending in a lengthwise direction from an inlet end toan outlet end of the mould, said cavity having a cross-section normal tothe lengthwise direction which comprises two rectangular side regions ofgenerally constant width and breadth separated by a central region ofgenerally constant width; at the inlet end of the cavity, the centralregion constitutes a pouring region and, at the pouring region, theopposite walls of the central region which separate the side regions areof arcuate form; and for at least part of the length of the centralregion in the direction towards the outlet end, the radii of curvatureof the arcuate walls progressively increase to reduce the breadth of thecentral region and constitute as deformation region which serves todeform a workpiece case in the mould as it moves therethrough in thedirection towards the outlet end.
 2. A continuous casting mould asclaimed in claim 1, wherein the radii of curvature of the arcuate wallsprogressively increase from a position in the lengthwise direction fromthe inlet end of the mould cavity.
 3. A continuous casting mould asclaimed in claim 1, wherein the opposite walls of the pouring region, atleast at the inlet end of the mould, include inserts of a materialhaving a lower thermal conductivity than the material from which theremaining parts of the mould are formed.
 4. A continuous casting mouldas claimed in claim 3, wherein the inserts are of a copper matriximpregnated with silicon nitride or boron nitride.
 5. A continuouscasting mould as claimed in claim 1, wherein the widthwise dimension ofthe mould cavity is adjustable.
 6. A continuous casting mould as claimedin claim 1, wherein means for oscillating the mould in the directionparallel to the length of the mould cavity are provided.
 7. A continuouscasting mould as claimed in claim 1, in combination with a containmentzone having inlet and outlet ends, the inlet end of the containment zonebeing positioned at the outlet end of the mould, said containment zonealso having a deformation region, and the deformation region of themould being contiguous to the deformation region of the containmentzone.
 8. The combination claimed in claim 7, wherein the radii ofcurvature of the walls of the workpiece in the deformation region in thecontainment zone increase to infinity at a position above the outlet endof the containment zone.
 9. A continuous casting mould as claimed inclaim 1, wherein, from a first position in the lengthwise direction fromthe inlet end of the mould cavity, the curvature of the arcuate wallsprogressively increase to infinity at a second position which is betweena first position and the outlet end of the mould cavity.